In printers, fan folded paper is commonly fed by paper moving devices located on opposite sides of a print station which in a dot matrix printer commonly comprises a horizontal bar platen and a traversing print head. The paper moving device on the upstream side of the print station often comprises pin feed devices such as pin belt tractors which engage perforations along opposite edges of the paper. The paper moving device on the downstream side is usually a friction feed device comprised of one or more pairs of cooperating pressure feed rolls. The paper feeding devices are operated in a coordinated manner by a common drive mechanism for feeding the paper in a substantially straight line path in forward and reverse directions across the platen and through the print station. Among other things the use of dual paper moving devices enables the paper, which can be either single or multiple layers, to be tensioned for printing. A tear structure may also be provided downstream from the feed rolls to allow, portions of the paper to be separated from the printer. Examples of printers with such paper feeding, are the IBM 4224 Printer and the IBM 4230 Printer. Other examples of dual feed devices of the above type are shown in U.S. Pat. Nos. 5,030,025; 5,005,746; 4,995,745 and 4,671,686, the latter describing the dual feed devices as parts of a paper feed module or assembly which can be removed from, and installed onto the frame of a printer. The printer frame contains the platen, the print mechanism and the power drive system along with electrical controls for operating the print mechanism and the power drive system.
In order to realize the maximum utilization of the paper, the leading edge margin should be as small as possible, i.e. the first print line should be as close as practicable to the leading edge of the paper. It has been found, however, that printing close to the, leading edge of certain grades of paper tends to cause the leading edge to curl or to spread in the case of multi-layer paper. Consequently, the leading edge of the paper, unless constrained as it is moved beyond the print station toward the feed rolls, will be misaligned and will not enter the feed rolls properly so that the paper jams or is damaged. Curling or spreading of the leading edge of the paper also occurs when the paper is torn off at the tear structure. Thus, the same problem of misalignment and paper jamming results when the paper, in order to print near the leading edge, is then reverse fed to the extent that the leading edge is no longer held within the feed rolls.
One possible approach to solving the problem would be to provide means for separating the feed rolls until the paper is within the pinch point or bite of the rolls. Another approach might be to use paper which is sufficiently stiff so that curling or spreading is minimal or non-existent. Both approaches have obvious limitations. A further approach is to provide a paper guide structure between the print station and the feed rolls. Heretofore, one type of guide structure consists of stationary guide plates arranged to form a channel or chute extending between successive pairs of feed rolls or between a rotating platen and a pair of feed rolls. Such structures have been primarily designed for single sheet feeding and are not easily adjusted to accommodate paper having different layers. Another type paper guide structure consists of one or more stationery guide plates extending through and in either direction beyond the print station and includes springs or spring loaded pressure rolls or belts which press the paper against a surface of the guide plate. Besides being unduly complex, such structures place limitations on the design and servicing of the printer and paper feed devices. Sheet feeding devices are known which use mechanisms for removing curl from the sheet to improve stacking. None have dealt with the problem of leading edge curling or spreading of single or multilayer papers.
Examples of various types of guide structures may be seen in U.S. Pat. Nos. 2,528,420; 2,973,957; 3,352,470; 3,556,512; 4,197,025; 4,669,721; 4,842,263; 4,903,954; 4,995,745; 4,995,746; 4,997,179; 5,000,598; and 5,030,024. U.S. Pat. No. 5,078,525 deals with the problem of distortion across the center section of the leading edge of a document being fed by pin feed tractors into the entry structure of a printer. The problem is solved by providing an entry plate with different entry slope angles to guide different parts of the distorted leading edge of the paper into the entry. None of these references is suitable for solving the leading edge curling/flaring problem when paper is being fed into feed rolls.